Generators without moving parts include, for example, a thermionic generator (TIG), a thermoelectric generator (TEG), a thermophotovoltaic generator (TPV) and/or a thermotunnel generator (TTG). The connection of generators can, for example, serve as a source of electrical energy.
A TIG comprises a diode having two electrodes, one of which is called the emitter and the other the collector, with therebetween a slotted gap which is a vacuum or which is filled with an ionisable gas. In order to become detached from the surface of the emitter, electrons have first to overcome a threshold tension known as the operating function of the electrode material. Owing to the magnitude of the operating function, electrons become detached from the emitter only at relatively high temperatures. The detached electrons are conveyed to the collector as a result of the fact that heat, in this case the kinetic energy of the electrons or ions, flows from the warm emitter to the colder collector. The electric charge of the electrons also produces an electric current.
However, as a result of the fact that the thermionic effect is effective only at temperatures above approximately 1,600 K, a large amount of radiation and conduction heat is also conveyed from the emitter to the collector and relatively high heat loss occurs. The maximum output obtained is thus 10 to 13%, and this is uneconomic for most applications. The use of the known converter is thus restricted to space travel and to applications in which a relatively low weight and long reliable availability are of crucial importance. If a multilayered TIG is used, the collector of one layer is connected to the emitter of the following layer, these connected electrodes forming a single entity.
A TEG comprises thermocouples made of “n” and “p”-doped semiconductor material, wherein an electric current flows in the “p” leg with the heat and in the “n” leg counter to the heat flow according to the Seebeck effect, and operates at temperatures of between 0 and 600 degrees Celsius and has outputs of up to 15%.
A TPV comprises a single-layered or multilayered diode which converts infrared radiation, emitted by a heat radiation emitter brought to high temperature, into an electric current. TPVs have, including the conversion output of the radiation emitter, outputs of up to 21% in the case of conversion of, for example, solar energy. In the case of the conversion of heat from a burner, outputs of up to 12% are obtained, provided that the residual heat in the outlet gases is recovered by a recuperator which preheats the inlet gases from the burner therewith.
A TTG operates like a TIG but can, owing to the tunnel effect, generate an electric current even at low temperatures of between 0 and 800° C. according to the thermionic effect. A TTG can obtain outputs of up to 40%.
One embodiment of a TPV is a micron-gap TPV (MTPV). The MTPV is irradiated by the heat radiation emitter at a very small distance of approx. 100 nm, whereas the space therebetween is evacuated. As a result of the narrow space, radiation resonance occurs and a higher total output of 30% is obtained, although lower radiation emitter temperatures from 1,000 to 1,200 K can also be utilised.
The TEG and the TTG operate at lower temperatures and can thus even more effectively be preceded by a TIG, increasing the common output above the outputs of the individual generators.
Research is being conducted into increasing the output of a TEG and it is expected that the current outputs of 15% can be increased to 30%. However, the TIG currently has outputs of 13% and should be able to operate much more effectively as a connected component. Theoretically, it is expected that the output of a TIG should be able to increase to 40%. However, at present, the TIG has a large number of drawbacks.
A major drawback of the TIG is the heat radiation between the electrodes, which cannot be converted into electrical energy. Solutions to the above-mentioned problem include, for example, the use of other emitters which are able to cope with higher energy density. This reduces the losses of heat radiation. Other possibilities include the use of a plurality of diodes, reducing the difference in temperature per layer and thus also the radiation losses.
Another major drawback of the TIG results from the fact that caesium gas is used in the gap to lower the operating function. The caesium gas is necessary to obtain a sufficiently high power density. The use of caesium gas gives rise to internal heat losses and current losses. The caesium is not necessary if the operating function is lowered in different ways, for example by the use of a thinner and subsequently thermally evacuated gap of from 100 to 2,000 nm, the use of a nanostructure comprising cones and/or grooves having a height of from 5 to 200 nm and the use of semiconductors.
The designing of a lower operating function also allows electrons to be emitted at lower temperatures (1,000-1,400 K). This allows a plurality of TIGs to be connected at lower intermediate temperatures and the above-mentioned radiation losses to be further reduced.
Another major drawback is the heat losses of electrons having higher energy than the electrical potential energy between the emitter and the collector and the plurality of heat conversions. Even reflective electrons which transfer their heat but not their charge give rise to losses. Known TIGs are cylindrical and/or dome-shaped. Thermal expansion makes it difficult to provide these TIGs with a plurality of layers. For a good output, the gap between each layer has to be precisely adjusted, as does the distance between the TIG and the generators connected to the TIG without moving components. However, all of these improvements to increase the output require the slot height of the gaps to be adjusted with uniform precision, and this cannot be achieved or is hardly achievable with the current embodiments.
When used in space travel, the energy converter is started up once and it is possible to eliminate temperature stresses which are produced. A larger market for energy converters without moving parts is the use of portable power supplies for replacing batteries. On account of the high energy content of the fuels such as diesel, these energy converters can, depending on the output, be 2 to 10 times lighter than conventional batteries. However, for this application, the converter has to be able to start and stop frequently, and alternating thermal stresses can be fatal owing to fatigue, cracking, in the case of fixed connections, and wear caused by, inter alia, seizing, in the case of sliding connections. This impairs electrical and thermal contacts, as a result of which the output deteriorates while the service life is limited. In this large market and in the future, once the anticipated high outputs have been achieved, even larger markets such as haulage and solar energy, the energy converter will have to be able to start and stop frequently, and this is not readily possible in the current coupled and connected generators and the aimed-for improved generators, owing to alternating internal mechanical stresses and wear in the event of possible friction between the connected generators.
Connecting various generators allows the output to be increased if the generators are operative in a temperature range which is different for each generator but nevertheless optimal. The subsequent generator then still converts the residual energy from the preceding generator into electrical energy.
In the case of generators comprising moving parts, this method has already been utilised. Examples include a gas turbine which operates at high temperature and precedes a current turbine operating at a lower temperature. The common output of this “‘STEG”’ unit is 60%, whereas the individual outputs of the turbine generators are between 30 and 40%.
A known converter comprising a connected combination of generators without moving parts comprises a TEG with a TPV, the heat being generated by combustion. The residual heat from the outlet is then converted by the TEG into electrical energy. As a result of the fact that the process is not much more cost-effective than the recovery of heat using an inexpensive recuperator, the output is increased—at much higher cost—by just 12% to 14%. A problem of the TPV is that the radiation emitter thereof operates at a high temperature of approx. 1,500° C. and that the TPV operates at a low temperature of from 25-50° C. No other generator can be connected between the radiation emitter and the TPV, and there are few possibilities for increasing the output by connecting to other generators. A converter comprising a TPV converts sunlight, which has first been concentrated, into heat by allowing the light to radiate onto a combined absorber/emitter. Subsequently, the radiation heat is converted by a TPV into electrical energy. The absorber/emitter is heated in this case on the sun side by absorbing the light and radiates on the TPV side heat radiation to the TPV. The problem of the absorber/emitter is that the emitter temperature drops as the sunlight diminishes. At a lower temperature, the radiation decreases but the wavelength also shifts to an area where the TPV is less sensitive, so the output decreases.
In other known converters, a thermionic generator (TIG) and a TEG are joined together, wherein the residual heat from the TIG, which has a temperature of approximately 900 K, can beneficially be used by the TEG, as may be found, inter alia, in U.S. Pat. No. 3,189,765. Problems with this include the fact that, as a result of the difference in thermal expansion, the TIG and the TEG make poor thermal contact owing to mechanical instability, such as wear and cracking, and the components can break down as a result of fatigue stresses if the connected generators are started up and stopped frequently. It is expected that the output will after just a few start-ups have deteriorated by 10% and will subsequently drop by 50%.
In other known converters, a multiplicity of TIG elements are connected electrically in series to generate higher electric tensions and lower currents, and these reduce the internal and external electrical losses, as may be found, inter alia, in U.S. Pat. Nos. 6,037,697, 3,432,690. In this case, electrical contact is established between the hot emitter of one TIG element and the relatively cold collector of the following TIG element. In the case of the known converters, the short distance gives rise to large heat losses and high fatigue stresses in the electrical connections between the TIG elements. On account of the short distance, thermal losses will lower the output by 10% and, after a plurality of start-ups, fatigue stresses will further impair the output by 20 to 30%.
In the case of gaps having a slot height of less than approximately 1 micrometer, caesium gas is no longer required and the losses of the TIG are somewhat lower. U.S. Pat. No. 6,411,007 and other documents utilise this by producing a TIG made by chemical vapour deposition (CVD). The small slot height of the gap is in this case maintained by spacer elements, the length of which corresponds to the slot height of the gap. These elements produce in this case temperature gradients of from approximately 108-109 K/m. Problems stemming from this include high thermal losses in the spacer elements and high fatigue stresses in the generators during starting and stopping. As a result of the high thermal losses, the output deteriorates by 20-40% and will deteriorate by 30-70% after a plurality of start-ups.